The present invention relates to a passive optical network (PON) system with a plurality of subscriber link devices sharing an optical transmission line.
The PON is generally made up of a single station-side apparatus, such as an optical line terminal (OLT), and a plurality of subscriber-side devices, such as optical network units (ONUs), wherein an optical signal from a terminal, such as a personal computer (PC) being connected to an ONU, is converted into an optical signal, which is then sent out toward the OLT via optical fiber or “fibre” cables. Optic fibers extending from more than two ONUs are coupled together by an optical multipoint splitter, and the optical signal is arrived at the OLT while being applied optical (time-division) multiplexing by this optical splitter.
The length of an optic fiber between ONU and OLT is defined in the ranges of 0 to 20 km, 20 to 40 km, and 40 to 60 km, for example, in clauses 8 and 9 of Recommendation G.984.1 of Telecommunication Standardization Sector of International Telecommunication Union (“ITU-T”). Each ONU is installed at a given distance from OLT, which falls within the ranges. Accordingly, transmission delay of an optical signal between OLT and each ONU is different depending upon an optic fiber length. If this transmission delay is not taken into consideration, it will possibly happen that optical signals as output from respective ONUs collide and interfere with each other during optical multiplexing at the optical splitter.
Consequently, an attempt is made to cause the OLT to adjust, by use of a specific technique called the “ranging” as set forth in clause 10 of ITU-T Recommendation G.984.3, the delay of an output from each ONU in such a way that each ONU is virtually situated at an equal position from the OLT, e.g., at a distance of 20 km therefrom, thereby to ensure that an optical signal from each ONU hardly interferes with the others.
In the ranging, the OLT requires ONUs to transmit a signal for the distance measurement use. When an ONU returns a distance measurement frame, OLT receives such signal and then measures the length of a time period of from the issuance of a distance measurement-use signal transmission request to reception of a distance measurement-use signal—that is, reciprocal delay time—to thereby know the exact distance between the ONU and OLT. Subsequently, in order to virtualize all ONUs so that these are at the equal distance, the OLT sends to each ONU an instruction which delays signal transmission by a certain length of time, called the equalized delay amount. For example, in order to force every ONU to have reciprocal delay time of 20 km, the OLT instructs to ONU the equalized delay amount that is equal to a subtraction value of the 20 km reciprocal delay time minus the measured reciprocal delay time. ONU has its circuitry for sending data while delaying it fixedly by the equalized delay amount instructed. In responding to receipt of the above-noted instruction, up-link or upload data transmission is performed in a way such that all the ONUs involved have the reciprocal delay time of 20 km.
Additionally, in order to equitably allocate the communication frequency band of a single optic fiber to a maximally increased number of ONUs in deference to requests from users, a dynamic bandwidth assignment (DBA) technique is also defined in ITU-T Rec. G.983.4, which is for permitting OLT to allocate a transfer channel band (data transmission position/time) in the upstream direction from each ONU. Frequency band control based on this technique is also performed.
For example, according to the definitions of clause 8.2 of ITU-T Rec. G.984.3, a signal to be transferred from more than two ONUs toward OLT is called the upstream signal, i.e., upload signal. This signal consists essentially of a preamble, a delimiter, and a payload signal. As shown in FIG. 8-2 in clause 8 of ITU-T Rec. G.984.3, a guard time is set immediately before each upload signal in order to prevent unwanted collision with its previous burst signal. Also note that according to the definitions of clause 8.1 of this ITU-T recommendation, a signal being sent from OLT to more than two ONUs is called the downstream signal—i.e., download signal. This signal consists essentially of a frame sync pattern, a physical layer operations/administration and maintenance (PLOAM) field, an upstream bandwidth map (USBWmap) field, and a frame payload.
As shown in clause 8.1.3.6 of ITU-T Rec. G.984.3, the OLT uses the USBWmap field to designate each ONU's upload signal transmission allowing timing. The USBWmap field has a start value that designates the start-up of transmission allowance and an end value that designates termination, wherein designation is performed in units of bytes, respectively. This value is also called the grant value in the meaning of allowing signal transmission. And, a difference between the end value and a start value next thereto is an upload signal absence or “null” field, which corresponds to the guard time stated above. It is noted here that two or more band allocation units, called the transmission containers (T-CONTs), are allocable to the individual ONU, causing the assignment of upload signal transmission grant timing to be performed with respect to each T-CONT.